For example, U.S. Pat. No. 7,080,551 B2 (JP-A-2005-172735) discloses a liquid measurement apparatus, which measures an amount of liquid flowing from a measured object such as an injector so as to check leakage of liquid from the measured object. The liquid measurement apparatus has a measurement passage, which is connected at one end with the measured object and filled with a test liquid A bubble is injected in the test liquid such that the bubble is movable in the measurement passage depending on change in amount of the test liquid in the measurement passage. The liquid measurement apparatus obtains images by photographing the bubble using a camera so as to acquire change in position of the bubble based on the images. Whereby, the liquid measurement apparatus measures the amount of liquid flowing from the measured object. Furthermore, in the liquid measurement apparatus, a diaphragm is provided in the measurement passage, so that the maximum travel distance of the bubble due to change in amount of liquid is limited to restrict the bubble from being deviated from a measurable range.
However, when such a liquid measurement apparatus is used over a long period, the test liquid increasingly vaporizes through an open end of the measurement passage, and consequently the bubble is gradually shifted toward the diaphragm. Therefore, the test liquid is repeatedly filled, and the bubble is also repeatedly injected in a relatively short period. Moreover, when vibration is applied to the liquid measurement apparatus, or environmental change such as temperature change occurs during the measurement, the bubble position may be shifted. As a result, measurement accuracy is impaired.